Frequency-domain terahertz generation

Continuous-wave (cw) terahertz radiation is obtained by so-called optical heterodyning in high-bandwidth semiconductors: The output of two cw lasers is converted into terahertz radiation, exactly at the difference frequency of the lasers.

The core component is the “photomixer”, a microscopic metal-semiconductor-metal structure. Near-infrared laser light irradiates this structure at two adjacent frequencies. Applying a bias voltage to the metal electrodes generates a photocurrent that oscillates at the beat frequency. An antenna surrounding the photomixer emits an electromagnetic wave – at the terahertz difference frequency. Benefits of this technique include high resolution, spectral selectivity, and superior dynamic-range values.

On the laser side, the systems include distributed-feedback (DFB) diodes. These diodes comprise a grating structure within the active section of the semiconductor, restricting the laser emission to a single longitudinal mode. Thermal tuning of the grating pitch yields very wide continuous frequency scans (typ. > 1000 GHz / diode). By selecting two DFB diodes with appropriate wavelength offset, one can tune the terahertz difference frequency continuously, e.g. from 0 to 2 THz.